This research seeks to develop techniques for the on-line measurement of the size-composition spectra of airborne particulates and to apply these techniques in the measurement of the atmospheric aerosol. Because the toxicity of inhaled particulates depends on both their size and composition, simultaneous measurement of both properties is essential to assessing the public health effects of atmospheric particulates of various kinds. Current techniques are not capable of on-line measurement of both size and composition. This research seeks to combine aerodynamic sizing methods and mass spectroscopy in an on-line system for the measurement of suspended particulates. The system, now nearly completed, will be used to examine the atmospheric aerosol to determine the extent of various chemical species, especially toxic and possibly toxic species, and their distribution in particle size and to determine features of the size-chemical nature of atmospheric particulates that may serve to infer their origins, history, toxicity and the presence of other less readily measurable species.